Paleomagnetism serves as an effective tool for studying continental drift, plate tectonics, and regional deformation, offering unique advantages in paleogeographic reconstructions and tectonic studies. However, over the lengthy geological history following rock formation, primary magnetic field information recorded in rocks may be overprinted by later magnetic fields or even completely altered, a process known as remagnetization. Failure to identify remagnetization can significantly affect paleogeographic reconstructions. Common field tests (such as fold tests, reversal tests, heating tests, and pebble tests) can help determine the primary nature of the paleomagnetic direction recorded in rocks. Additionally, studies on the remanent magnetization mechanisms using rock magnetism can provide important evidence for identifying remagnetization.
Recently, Dr. Yan Maodu and his colleagues from the Institute of Tibetan Plateau Research, Chinese Academy of Sciences, conducted a systematic comparison of rock magnetism in two sets of rocks from the Zado area of Yushu Prefecture, Qinghai Province (Figure 1). One set comprised rocks with remagnetization, including Upper Jurassic Yanshiping Formation limestone and Lower Carboniferous Zado Formation limestone, while the other set consisted of rocks with primary remanent magnetization, including Late Permian-Early Triassic Gadika Formation volcanic rocks and Early Cretaceous (126 Ma) granite. The main magnetic mineral in these four rock sets is magnetite. Stable single-domain (SD) and pseudo-single-domain (PSD) grains dominate in the remagnetized rock samples, while SD grains prevail in the primary remanent magnetization samples. The decay curves of natural remanent magnetization (NRM) intensity show that the unblocking temperature of secondary magnetite is lower than that of primary magnetite. Analysis of isothermal remanent magnetization (IRM) endmembers and component analysis revealed the presence of SP magnetite in the remagnetized rock samples. Low-field and high-field thermal demagnetization curves both indicate the transformation of iron sulfides to magnetite during heating. Additionally, hysteresis characteristic analysis revealed significant differences between the two types of magnetite; for example, remagnetized rock samples exhibit a waist-shaped hysteresis loop due to the combination of SP and SD magnetite, whereas primary remanent magnetization rock samples mainly display a bulb-shaped loop. Furthermore, differences between remagnetized and primary remanent magnetization rocks were assessed and compared using various magnetic domain state diagrams, including Day diagrams, Néel diagrams, Borradaile diagrams, and Fabian diagrams (Figure 2). In Day diagrams, the remagnetized rock samples plot closer to the SP-SD trend line, while the primary remanent magnetization rock samples cluster near the SD-MD trend line. In Néel diagrams, remagnetized rock samples are distributed on the left side of the USD-SP region, whereas primary remanent magnetization rock samples are more concentrated in the USD-SP region. However, no obvious differences between the two types of rocks are observed in Borradaile diagrams. Both Fabian diagrams and quantitative analysis of hysteresis loops reveal a consistent shape parameter; in most remagnetized rock samples, this parameter is greater than 0, while in most primary remanent magnetization rock samples, it is less than 0 (Figure 3). The effectiveness of distinguishing remagnetization based on hysteresis characteristics is demonstrated by analyzing a large number of hysteresis loops in the literature.
The study emphasizes that comprehensive rock magnetism research, especially involving Day diagrams, Fabian diagrams, thermal demagnetization decay curves, IRM component analysis, and endmember analysis, can be used alone or in combination with field test methods to determine the primary nature of the remanent magnetization direction in magnetite-bearing rocks.
This research, titled "Remagnetization of magnetite-bearing rocks in the eastern Qiangtang Terrane, Tibetan Plateau (China): Mechanism and Diagnosis," was recently published in the renowned international journal Physics of the Earth and Planetary Interiors. Dr. Qiang Fu, a graduate of the institute, is the first author, while Dr. Yan Maodu is the corresponding author. This study was jointly supported by the National Natural Science Foundation of China (41974080, 41988101-01), the National Key R&D Program of China (2022YFF0800502), and the Special Project on the Second Tibetan Plateau Scientific Expedition and Research (2019QZKK0707).
Paper link: https://doi.org/10.1016/j.pepi.2024.107184
Figure 1: Overview of Geological Setting and Profiles of the Study Area
Figure 2: Selected Rock Magnetic Results
Figure 3: Quantitative Analysis of Hysteresis Loop
